I wouldn't dream of sailing without specs. For one thing i wouldn't be able to find my boat on the beach even with the sails up!
I find that they protect my eyes from driving spray at speed and i can keep looking at the telltales/bows when 'two-eyed' folks have to shut them!
I just tie a very thin piece of shock cord across the back of the glasses and they have only ever had them come off once, that was when i mannaged to jam my head through the purchases of my main sheet on a gybe! even then they were safe around my neck.
I do always have a spare set taped to the boom just in case a lense falls out or something.
FOR GODS SAKE DON'T LET GLASSES PUT YOUR SON OFF OF THE SPORT.

Paul 4-eyes